EE6702 -

PROTECTION &

SWITCHGEAR

By,

VINITHA JC

ASSO. PROF.RMD-EEE

UNIT 2 - RELAY

UNIT 2 SYLLABUS

• Electromagnetic relays

• over current relay

• directional relay

• non-directional relay

• distance relay

• negative sequence relay

• differential relay

• under frequency relay

• Introduction to static relays

RELAY OVERVIEW

WHAT ARE RELAYS?• Relays are electrical

switches that open or

close another circuit

under certain

conditions.

RELAY PURPOSE

• Isolate controlling circuitfrom controlled circuit.

• Control high voltage systemwith low voltage.

• Control high current systemwith low current.

• Logic Functions.

HOW A RELAY WORKS?

ELECTROMAGNETIC

RELAY

They work on the following

two main operating principles

:

(i) Electromagnetic

attraction

(ii) Electromagnetic

induction

ELECTROMAGNETIC ATTRACTION RELAYS

(i)Attracted armature type relay

(ii)Solenoid type relay

(iii)Balanced beam type relay

• (i)Induction type overcurrent Relay(Non Directional Relay)

• (ii)Induction Cup Relay (Directional Relay)

Induction Relays / Electromagnetic induction

1. ATTRACTED ARMATURE TYPE RELAYS

• These have a coil orelectromagnet energized by acoil .The coil is energized byoperating quantity like V or I.

• Under normal conditions the coilcannot attract the plunger dueto spring force. Under faultcondition the fault currentincreases so armature orplunger gets attracted to closethe contacts .

1. ATTRACTED ARMATURE TYPE RELAYS

1. ATTRACTED ARMATURE TYPE RELAYS

Applications

1.For over currentprotection

2.Differential Protection

3.Auxiliary Relays

4.Definite time lag overcurrent and earth faultprotection

2. SOLENOID TYPE RELAY

• It consists of a solenoid and movable iron plunger arrangedas shown.

• Under normal operating conditions, the current through therelay coil C is such that it holds the plunger by gravity orspring in the position shown.

• However, on the occurrence of a fault, the current throughthe relay coil becomes more than the pickup value, causingthe plunger to be attracted to the solenoid. The upwardmovement of the plunger closes the trip circuit, thusopening the circuit breaker and disconnecting the faultycircuit.

2. SOLENOID TYPE RELAY

3. BALANCED BEAM TYPE RELAY

It consists of an iron armature fastened to a balance

beam. Under normal operating conditions, the current

through the relay coil is such that the beam is held in

the horizontal position by the spring.

When a fault occurs, the current through the relay coil

becomes greater than the pickup value and the beam

is attracted to close the trip circuit. This causes the

opening of the circuit breaker to isolate the faulty

circuit.

3. BALANCED BEAM TYPE RELAY

INDUCTION CUP

STRUCTURE

INDUCTION CUP STRUCTURE

It most closely resembles an induction motor, except

that the rotor iron is stationary, only the rotor

conductor portion being free to rotate.

The moving element is a hollow cylindrical rotor which

turns on its axis. The rotating field is produced by two

pairs of coils wound on four poles as shown.

The rotating field induces currents in the cup to

provide the necessary driving torque.

INDUCTION CUP STRUCTURE

• If φ1 and φ2 represent the fluxes produced by therespective pairs of poles, then torque produced isproportional to φ1 Φ2 sinα.

• Where α is the phase difference between the two fluxes. Acontrol spring and the back stop for closing of the contactscarried on an arm are attached to the spindle of the cup toprevent the continuous rotation.

• Induction cup structures are more efficient torqueproducers than either the shaded-pole or the watthourmeter structures. Therefore, this type of relay has very highspeed and may have an operating time less then 0.1 second.

INDUCTION TYPE

OVERCURRENT RELAY

(NON DIRECTIONAL

RELAY)

INDUCTION TYPE OVERCURRENT RELAY

(NON DIRECTIONAL RELAY)

• This type of relay works on the inductionprinciple and initiates corrective measures whencurrent in the circuit exceeds the predeterminedvalue.

• The actuating source is a current in the circuitsupplied to the relay from a current transformer.These relays are used on a.c. circuits only andcan operate for fault current flow in eitherdirection.

INDUCTION TYPE OVERCURRENT RELAY

(NON DIRECTIONAL RELAY)

CONSTRUCTIONAL DETAILS

It consists of a metallic (aluminium) disc which is free to

rotate in between the poles of two electro magnets. The

upper electro magnet has a primary and a secondary

winding. The primary is connected to the secondary of a

C.T. in the line to be protected and is tapped at intervals.

The tappings are connected to a plug-setting bridge by

which the number of active turns on the relay operating coil

can be varied, there by giving the desired current setting.

The secondary winding is energized by induction from

primary and is connected in series with the winding on the

lower magnet. The controlling torque is provided by a spiral

spring.

The spindle of the disc carries a moving contact which

bridges two fixed contacts (connected to trip circuit) when

the disc rotates through a pre-set angle. This angle can be

adjusted to any value between 0° and 360°. By adjusting

this angle, the travel of the moving contact can be adjusted

and hence the relay can be given any desired time setting.

OPERATION

• The driving torque on the aluminium disc is set up due to theinduction principle. This torque is opposed by the restrainingtorque provided by the spring.

• Under normal operating conditions, restraining torque is greaterthan the driving torque produced by the relay coil current.Therefore, the aluminium disc remains stationary.

• If the current in the protected circuit exceeds the pre-set value,the driving torque becomes greater than the restraining torque.Consequently, the disc rotates and the moving contact bridges thefixed contacts when the disc has rotated through a pre-set angle.The trip circuit operates the circuit breaker which isolates thefaulty section.

DIRECTIONAL RELAY

INDUCTION RELAYS

induction relays are two types

non directional relays directional relays

Difference between the two:…………?

• non directional relays are activated by only current flowing inthe circuit to be protected.

• directional relays are activated by power flowing in thespecific direction. Hence it requires both current andvoltage of the circuit to be protected.

* it requires specific direction of current flow*

DIRECTIONAL POWER RELAY

DIRECTIONAL POWER RELAY

Constructional details:

• It consists of two electro magnets

1) upper magnet which is E-shaped

2) lower magnet which is U-shaped.

• The upper magnet consists of primary winding onthe central limb which is energized by voltagefrom secondary of P.T

• lower magnet houses secondary winding which isenergized by current of the circuit from secondaryof C.T.

Further lower magnet is connected to PSMas previous case (not shown)

DIRECTIONAL POWER RELAY

• In between this two electro magnets we have aluminum discpivoted as shown

• This aluminum disc carries a moving contact which can bridgefixed contact by rotating though a pre set angle.

• The time of operation depends upon the pre set angle

• Restraining torque is provide by spring which twists in reversedirection.

Operation:

• from the diagram we can conclude that we have two flux quantaties: φ1 & φ2 .

always φ1 laggs V by 90 0

φ2 inphase with current I

DIRECTIONAL POWER RELAY

• Due to phase difference between two flux quantities i.e., α=

90-θ

Φ1 αV & φ2 αI

Hence T = φ1 φ2 sin α

= φ1 φ2 sin(90-θ)

= VI COSθ

= POWER

• Hence the relay activated only when there is a specific

direction of power flow.

• when power flows in normal direction both driving torque

and restraining torque twists in same direction and relay

does not operates.

• when the power flow is in reverse direction, driving torque

and restraining torque acts in opposite direction and relay

operates. therefore CB operates and disconnects faulty

section.

DIRECTIONAL OVER CURRENT RELAY

• From the previous discussion

T = V I COSθ

Under abnormal condition

under abnormal conditions voltage in the circuit is toolow. Therefore the driving torque becomes abnormally toosmall. Hence the relay does not operate.

ie., the directional power relay is not suitable for shortcircuit conditions.

This problem can be overcome by directional overcurrent relay.

DIRECTIONAL OVERCURRENT RELAY

DIRECTIONAL OVERCURRENT RELAY

• Directional overcurrent relay makes use of two relays

1) directional power relay ( directional element)

2) Non directional current relay (non-directional element)

Construction:

1)Directional element :– It is similar in construction to directional power relay.

• It consists of upper magnet which is E-shaped and carries primary winding which isexcited by voltage of the circuit to be protected through secondary of PT.

• The lower magnet is U-shaped carries secondary winding which is excited by current ofthe circuit to be protected through secondary of CT.

• The secondary winding is extended to lower magnet primary winding as shown.

• The trip contacts 1 & 2 are connected in series with secondary winding of lower magnet.

therefore for the relay to operate, at first directional element should be activatedfirst.

DIRECTIONAL OVERCURRENT RELAY

2) Non directional element:

* It is activated only by current flowing in the circuit*

• it is similar in construction to non-directional over current relay. Forthis element to operate ,at first directional element should beactivated first.

• the secondary winding is further connected to PSM( not shown), forcurrent setting.

Operation :

• When short circuit occurs current tend to be reversed. Hencedirectional element starts operating and closes the trip contact.

• with closing of trip contact, the secondary winding of non directionalelement is complete and disc starts rotating. When moving contactbridges fixed contact the circuit breaker operates and separates thefaulty section.

DIRECTIONAL OVERCURRENT RELAY

DISTANCE RELAY(MHO

RELAY)

1. IMPEDANCE RELAYDistance relay :

1.IMPEDANCE RELAY

• +ve (operative)Torque by current element

• -ve(restraining)Torque by voltage element

• At normal condition

operative torque = restraining torque

• At faultoperative torque > restraining torque

• Also called voltage restrained over current relay.

1. IMPEDANCE RELAY

1. IMPEDANCE RELAY

2. REACTANCE RELAY Operative Torque by current

Restraining Torque by Current-Voltage Directional

relay

+ve torque by over current element

-ve torque by directional unit

Directional element designed for maxi. Torque angle =

90 degree

2. REACTANCE RELAY

2. REACTANCE RELAY

3. MHO RELAY• Induction cup type structure.

• Operative Torque produced by V & I element.

• Restraining Torque by Voltage element.

• Also called Admittance relay.

3. MHO RELAY

3. MHO RELAY

NEGATIVE

SEQUENCE

RELAYS

NEGATIVE SEQUENCE RELAYS

• The negative relays are also called phaseunbalance relays because these relaysprovide protection against negativesequence component of unbalancedcurrents existing due to unbalancedloads or phase-phase faults.

• The unbalanced currents are dangerousfrom generators and motors point of viewas these currents can cause over heating.Negative sequence relays are generallyused to give protection to generators andmotors against unbalanced currents.

NEGATIVE SEQUENCE RELAYS

DIAGRAM

CONSTRUCTION

• It consists of a resistance bridgenetwork.

• The magnitudes of the impedances of allthe branches of the network are equal.

• The impedances Z1 and Z3 are purelyresistive while the impedances Z2 and Z4are the combinations of resistance andreactance.

• The currents in the branches Z2 and Z4lag by 60 degree from the currents in thebranches Z1 and Z3.

• The vertical branch B-D consists ofinverse time characteristics relay. Therelay has negligible impedance.

PHASOR DIAGRAM

NEGATIVE SEQUENCE RELAYS

The current IRgets divided into two equal parts I1and I2. And I2lags I1by 60o.

Ī1+ Ī2=Īrs

Let I1= I2= I

The perpendicular is drawn from point A on the diagonal meeting it at point B. This bisects the diagonal.

HENCE, OB = IR/2

Now in triangle OAB,

cos30 = OB/OA

HENCE, √3/2 = (IR/2)/I

HENCE, I = IR/√3 = I1= I2 ............(1)

NEGATIVE SEQUENCE RELAYS

• Now I1 leads IR by 30 degree while I2 lags IR by 30

degree.

• Similarly the current IB gets divided into two equal

parts I3 and I4. The current I3 lags I4 by 60 degree.

From equation(1) we can write,

IB/√3=I3=I4...............(2)

• The current I4 leads by IB while current I3 lags IB by

30 degree.

• The current entering the relay at the junction point B

in the Fig.1 is the vector sum of, and.

Irelay =Ī1+Ī3+ĪY

=IY+(IR/√3)(leadsIRby30degree)+IB/√3(lagsIBby3

0degree)

• When the load is balanced and no negative

sequence currents exist.

NEGATIVE SEQUENCE RELAYS

Ī1+ Ī3= -ĪYHence, Ī1+ Ī3+ ĪY= 0

Thus the current entering the relay at point B is zero. Similarly the resultant current at junction D is also zero. Thus the relay is inoperative for a balanced system.

CONDITION

Now consider that there is unbalanced load on

generator or motor due to which negative sequence

currents exist.

The component I1and I3 are equal and opposite to

each other at the junction point B. Hence I1and

I3cancel each other. Now the relay coil carries the

current IY and when this current is more than a

predetermined value, the relay trips closing the

contacts of trip circuit which opens the circuit breaker.

ZERO SEQUENCE CURRENT

• Under zero sequence currents the total current of twice thezero sequence current flows through the relay. Hence therelay operates to open the circuit breaker.

• To make the relay sensitive to only negative sequencecurrents by making it inoperative under the influence ofzero sequence currents is possible by connecting the currenttransformers in delta .Under delta connection of currenttransformers, no zero sequence current can flow in thenetwork.

DIFFERENTIAL

RELAYS

DIFFERENTIAL RELAYS

Definition

• A two-winding relay that operates

when the difference between the

currents in the two windings reaches a

predetermined value is called

differential relays.

• A two-winding relay that operates

when the difference between the

currents in the two windings reaches a

predetermined value.

DIFFERENTIAL RELAYS• In case of electrical quantities exceed a predetermined value, a

current differential relay is one that compares the current entering asection of the system with current leaving the section.

• Under normal operating conditions, the two currents are equal but assoon as fault occurs, this condition no longer applies. The differencebetween the incoming and outgoing currents is arranged to flowthrough relay operating coil. If this difference is equal to or greaterthan the pick up value the relay will operate and open the circuitbreaker and isolate the faulty section.

• Any type of relay when connected in a particular way can be made tooperate as a differential relay. It is not the relay construction but theway in which relay is connected in a circuit makes it a differentialrelay.

DIFFERENTIAL RELAYS

There are three fundamental systems of differential or balanced

protection:

1. current differential relay

2. voltage differential relay

3. Biased beam relay or percentage differential relay

(i) Current balance protection

Fig 16 a shows an arrangement of an over current relay connected

to operate as a differential relay. A pair of identical current

transformers is fitted on either end of the section to be protected

(alternator winding in this case). The secondary's of CT’s are

connected in series in such a way that they carry the induced

currents in the same direction. The operating coil of over current

relay is connected across the CT secondary circuit. This differential

relay compares the current at the two ends of the alternator

winding.

1. CURRENT DIFFERENTIAL RELAY

Under normal operating conditions, suppose the alternator

winding carries a normal current of 1000 A. Then the current in

the two secondary's of CT’s are equal as in figure. These currents

will merely circulate between the two CT’s and no current will

flow through the differential relay as shown in the diagram fig 16

a. Therefore, the relay remains inoperative.

If a ground fault occurs on the alternator winding as shown in fig

16 b. the two secondary currents will not be equal and the current

flows through the operating coil of the relay, causing the relay to

operate. The amount of current flow through the relay will

depend upon the way the fault is being fed.

DIS-ADVANTAGES

• The impedance of the pilot cables generally causes a slight

difference between the currents at the two ends of the

section to be protected, then the small differential current

flowing through the relay may cause it to operate even

under no fault conditions.

• Pilot cable capacitance causes incorrect operation of the

relay when a large current flows.

• Accurate matching of current transformers can not be

achieved due to pilot circuit impedance.

2. VOLTAGE DIFFERENTIAL RELAY

Fig. 18 shows the arrangement of voltage

balance protection.

• In this scheme of protection, two similar

current transformers are connected at either

end of the element to be protected (e.g. an

alternator winding) by means of pilot of wires.

• The secondary's of current transformers are

connected in series with a relay in such a way

that under normal conditions, their induced

e.m.f’s are in opposition.

2. VOLTAGE DIFFERENTIAL RELAY

• Under healthy conditions, equal currents will flow in bothprimary windings. Therefore, the secondary voltages of thetwo transformers are balanced against each other and nocurrent will flow through the relay-operating coil.

• When a fault occurs in the protected zone, the currents inthe two primaries will differ from one another and theirsecondary voltages will no longer be in balance.

• This voltage difference will cause a current to flow throughthe operating coil of the relay, which closes the trip circuit.

DIS-ADVANTAGES

The voltage balance system suffers

from the following drawbacks

• A multi-gap transformer

construction is required to achieve

the accurate balance between

current transformer pairs.

• The system is suitable for protection

of cables of relatively short, lengths

due to the capacitance of pilot wires.

3. BIASED BEAM RELAY OR PERCENTAGE

DIFFERENTIAL RELAY

The biased beam relay also called percentage differential relay is

designed to respond to the differential current in terms of its

fractional relation to the current flowing through the protected

section.

It’s called percentage differential relay because the ratio of

differential operating current to average restraining current is a

fixed percentage.

It’s called bias relay because restraining known as biased coil

produces the bias force. Fig 17 a, shows the schematic

arrangements of biased beam relay. It is essentially an over

current balanced beam type relay with an additional restraining

coil. The restraining coil produces a bias force in the opposite

direction to the operating force.

3. BIASED BEAM RELAY OR PERCENTAGE

DIFFERENTIAL RELAY

Under normal and through load conditions, the bias

force due to restraining coil is greater than operating

force. Therefore, the relay remains inoperative.

3. BIASED BEAM RELAY OR PERCENTAGE

DIFFERENTIAL RELAY

When an internal fault occurs, the operating force

exceeds the bias force. Consequently the trip contacts

are closed to open the circuit breaker.

The bias force can be adjusted by varying the number

of turns on the restraining coil.

INTRODUCTION TO

STATIC RELAY

STATIC RELAY

• The static relay is the next generation relay afterelectromechanical type.

• The Solid Static relays was first introduced in 1960’s. Theterm ‘static’ implies that the relayhas no movingmechanical parts in it.

• Compared to the Electromechanical Relay, the Solid Staticrelay has longer life-span, decreased noise whenoperates and faster respond speed.

• The static relays have been designed to replace almost allthe functions which were being achieved earlier byelectromechanical relays.

PRINCIPLE OF OPERATION

The essential components of static relays are

shown in figure below. The output of CT and

PT are not suitable for static components so

they are brought down to suitable level by

auxiliary CT and PT. Then auxiliary CT

output is given to rectifier.

Rectifier rectifies the relaying quantity i.e., the

output from a CT or PT or a Transducer.

PRINCIPLE OF OPERATION

• The rectified output is supplied to a measuring unitcomprising of comparators, level detectors, filters, logiccircuits.

• The output is actuated when the dynamic input (i.e.,the relaying quantity) attains the threshold value. Thisoutput of the measuring unit is amplified by amplifierand fed to the output unit device, which is usually anelectromagnetic one.

• The output unit energizes the trip coil only when relayoperates.

ADVANTAGES OF SOLID STATE RELAY

• Low Weight

• Arc less switching

• Static Relay burden is less than electromagnetic type ofrelays. Hence error is less.

• Fast response.

• Long life

• Less power consumption

• More Accurate compared to electromechanical Relay

THANK YOU